In conventional dentistry, when casting a dental restoration, in particular a model casting or crowns and bridges, a model is created using wax or a photo-curable plastic from a previously created positive model, representing the oral situation of the patient or a part thereof. The three-dimensional proportions of the wax or plastic model are therefore identical to those of the dental restoration to be created. The wax or plastic model is normally embedded with an embedding compound and burned away once the embedding compound has hardened, so that a casting mold is obtained. Then the desired casting material is cast into the casting mold produced, such that once the casting material has cooled the dental restoration is obtained.
New developments in dentistry use CAD/CAM technology. As a result the design of the wax or plastic modes is no longer based on a positive model, as described above, but takes place virtually. In order to obtain three-dimensional, digital data, normally to begin with either the dentition or a part of the dentition of the patient is scanned in the mouth of the patient or initially an impression is taken of the dentition or a part of the dentition of the patient, which is then scanned. On the basis of the data set obtained a wax or plastic model is then normally created by printing or stereolithography, in order to obtain a model which in terms of the physical proportions corresponds to the dental restoration to be produced, and which then (as a working model) according to the conventional method outlined above is surrounded with an embedding compound, in order that once the wax or plastic model has been removed (normally by burning off) a casting mold is obtained, which is cast with a casting material, in order to obtain the desired dental restoration. Such methods are described, inter alia, in the following publications: U.S. Pat. No. 7,463,942B2, U.S. Pat. No. 7,383,094B2, U.S. Pat. No. 669,134B2, U.S. Pat. No. 6,915,178B2, U.S. Pat. No. 6,957,118B2, DE 3003435A1, [Schweiger: “Rapid Prototyping—Technik der Zukunft?”, Dental-labor, 7 (2004), p. 1109] and [Schweiger: “Rapid Prototyping-Neue Fertigungswege in der Zahntechnik und Zahnmedizin”, DIGITAL_DENTAL.NEWS, 2 (2008), p. 36].
In the prior art it known in the production of cast parts for dentistry to use a refractory embedding compound, the expansion values of which compensate for the unavoidable contraction of the casting materials used (during cooling of the casting material after pouring into the casting mold).
After casting, as the hardened casting material cools, the cast parts become smaller due to the thermal expansion of the solid casting material, and possible changes in the crystalline structure. As a result of the contraction of the casting material during cooling of the hardened cast part, the original dimensions change and a so-called “solid shrinkage” takes place. In the context of the present invention the term “solid shrinkage” thus means the shrinkage that takes place at room temperature upon cooling after the hardening temperature of the casting material or the solidus temperature of a metal alloy used as a casting material has been reached. As a rule this differs according to the casting material. The amount of solid shrinkage of certain casting materials is normally given as a contraction dimension, according to the substance by the linear expansion coefficients α (also known as the linear coefficient of thermal expansion or thermal expansion).
In the prior art attempts are made to counter the contraction of the casting material, that is to say the solid shrinkage, by an expansion of the embedding compound that compensates (in part or in full) for the contraction. In order to control the expansion values of the embedding compound quartz or another modification of crystalline SiO2 (e.g. cristobalite) is often added to this. The expansion characteristics of the embedding compound can further be controlled through the nature and concentration of the blending liquid used for blending the embedding compound, in particular by the nature and concentration of the silicon dioxide nanoparticles that it contains. The overall expansion of the embedding compound up until the casting mold resulting from the hardened embedding compound is normally made up of the setting expansion, i.e. the expansion of the embedding compound as it hardens, and possibly also the thermal expansion of the embedding compound, which as a rule occurs when an embedded wax or plastic model is burned out. The embedding compounds used are accordingly normally designed in such a way that through their setting expansion and possibly the thermal expansion they are able to compensate for the solid shrinkage of the casting material as it cools (generally to room temperature). So normally, depending on the casting material used, a suitable embedding compound and the associated blending liquid are selected.
However, such an approach also comes with disadvantages. Thus, as described above, in such compounds crystalline SiO2 is often used, in order to achieve the required thermal expansion capacity. When quartz or other modifications of crystalline SiO2 are used, however, there is a risk, in particular after many years of unprotected exposure in a dental laboratory, of silicosis or even lung cancer.
Furthermore, the setting expansion in particular of the embedding compound normally used is subject to major variations. And as a result this behaves differently (inter alia as a function of the type and quality of the binder used), in particular under differing conditions during mixing (e.g. with regard to the processing temperature, mixing time and speed). Here it is the case that the greater the amount of the maximum setting expansion of the embedding compound to be achieved, the higher the absolute variation.
A further disadvantage of such an approach is that for various casting materials with different fixed variations as a rule in each case different combinations of embedding compound and blending liquid must be selected. Thus when the casting material to be used is changed as a rule the embedding compound and/or blending liquid must be changed, which can considerably slow or adversely affect the working or production process.
In addition, for phosphate bonded embedding compounds in particular, it is not possible to produce dental restorations with differing indications in a single casting mold and at the same time have a good fit. Thus for example a “6×” telescope and an anterior tooth telescope must be embedded and cast separately from one another (with different blending liquid concentrations).
A further challenge in the production of tailor-made dental restorations is the result of distortions due to the uneven hardening of the casting material during cooling after pouring this into the casting mold. Thus for example thinner areas cool more quickly than thicker ones. In the prior art no standard procedure is described for avoiding such distortions during casting. Occasional attempts at solving this problem are for example referred to by the term “core embedding”. A disadvantage of this method, however, is that two different embedding compounds are required and have to be mixed. Furthermore, the production process is slowed by the fact that the second embedding compound can only be added once the first embedding compound has hardened or set.
As a result of all this the need arises for an improved method for producing tailor-made dental restorations, preferably by CAD casting, wherein the abovementioned disadvantages can be avoided or at least reduced.